The present invention relates to a method and an apparatus for determining a constant current limit value by means of which a first current which flows through a battery cell is limited.
It is known from the prior art that battery cells have to be protected against excessively high and excessively low voltage levels since otherwise the service life of the said battery cells is shortened and the battery cells may, under certain circumstances, be irreversibly damaged. Constant current limit values for a first current which flows through a battery cell of this kind are determined for this reason. The said constant current limit values ensure that, when they are complied with, a voltage value of a first voltage remains between suitable voltage limit values. When the first current is flowing, the first voltage is applied between two terminals of the battery cell with which contact can be made from outside the battery cell. The constant current limit values are each determined by means of a first function which specifies a first profile of the first current which flows through the battery cell, the said first profile being calculated depending on a plurality of parameters by means of a calculation model. In this case, the calculation model is dependent on a predefined equivalent circuit which is used for the battery cell. The plurality of parameters include the first voltage, a second voltage which specifies a no-load voltage of the battery cell, a time variable and further parameters. The time variable specifies a variable prediction period which is also called a variable prediction horizon. In order to determine a constant current limit value, an appropriate voltage limit value for the first voltage and a predefined time value for the time variable are used in the first function. In this case, the predefined value of the time variable, also called the “prediction time”, specifies a duration of a maximum prediction period for which the determined constant current limit value is valid and at the end of which the first voltage reaches the corresponding voltage limit value when the first current is limited by means of the determined constant current limit value. The further parameters are dependent on a structure of the predefined equivalent circuit. The constant current limit values specify a maximum available constant current in the discharge direction during the maximum prediction period, and a maximum available constant current in the charging direction during the maximum prediction period. Determining a constant current limit value in this way is also known from document DE 10 2012 204 957 A1.
Document JP 2010-169609 A discloses a battery comprising a first determination unit and a second determination unit. The first determination unit is designed to determine a plurality of parameters of a predefined equivalent circuit which is used for the battery. In this case, the plurality of parameters are determined on the basis of measured voltage values of a battery voltage which is applied between two terminals of the battery and on the basis of measured current values of a battery current which flows through the battery. The plurality of parameters specify resistance and capacitance values of resistors and capacitors which are found in the predefined equivalent circuit. Furthermore, the first determination unit is designed to carry out correction of a parameter from amongst the plurality of parameters on the basis of a deviation. In this case, the deviation specifies a difference between an actually measured voltage value of the battery voltage and a voltage value of the battery voltage which is calculated by means of a calculation model which is dependent on the parameters. The second determination unit is designed to determine a constant current limit value for the battery current depending on the plurality of parameters.